JP7610550B2 - Particles convertible into lithium transition metal composite oxides and method for producing same - Google Patents

Description

The present invention relates to particles that can be converted into lithium transition metal composite oxides. The present invention also relates to a method for producing the particles.

In recent years, lithium-ion secondary batteries have been used effectively as portable power sources for personal computers, mobile terminals, etc., and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

In general lithium ion secondary batteries, lithium transition metal composite oxides are used as the positive electrode active material. Examples of methods for producing lithium transition metal composite oxides include a method of preparing a precursor hydroxide containing no lithium but a transition metal, mixing this precursor with a lithium salt, and firing the resulting mixture (see, for example, Patent Documents 1 and 2), a method of preparing a precursor carbonate containing no lithium but a transition metal, mixing this precursor with a lithium salt, and firing the resulting mixture (see, for example, Patent Documents 3 and 4), and a method of granulating a lithium-free hydroxide containing no lithium but a transition metal and a lithium salt by spray drying, and firing the resulting granulated particles (see, for example, Patent Document 5).

JP 2021-153058 A JP 2021-180191 A JP 2019-21425 A JP 2021-136096 A JP 2018-129140 A

However, as a result of intensive research by the present inventors, it was found that the above-mentioned conventional technology has a problem in that when the obtained lithium transition metal composite oxide is used as a positive electrode active material in a lithium ion secondary battery, the resistance of the lithium ion secondary battery is not sufficiently reduced.

Therefore, the present invention aims to provide particles that can be converted into lithium transition metal composite oxides that can reduce the initial resistance of lithium ion secondary batteries.

The particles disclosed herein are particles that are converted into a lithium transition metal composite oxide by sintering. The particles contain lithium and a transition metal element within each particle. The average particle diameter of the particles is 2.0 μm or more. When a backscattered electron image of the particles is obtained using a scanning electron microscope, the maximum diameter of the domain that is recognized as the brightest area within the particle is less than 1 μm.

The method for producing particles disclosed herein includes the steps of: preparing a mixed solution in which a water-soluble salt of lithium, a water-soluble salt of a transition metal, an ammonium ion donor, and a basic compound are dissolved;
and precipitating particles containing lithium and a transition metal from the mixed solution.
(a) the basic compound is a hydroxide of an alkali metal or an alkaline earth metal, and the molar ratio of lithium to the transition metal in the mixed solution is 65 or more and 110 or less;
(b) the basic compound is a carbonate of an alkali metal or an alkaline earth metal, and the molar ratio of lithium to the transition metal in the mixed liquid is 5.0 or more and 8.5 or less, or (c) the basic compound is a carbonate of an alkali metal or an alkaline earth metal, and in the step of precipitating the particles, a water-soluble organic solvent having a solubility parameter δ of 13.0 or less and P' of 3.5 or more is added to the mixed liquid.

This configuration makes it possible to provide particles that can be converted into lithium transition metal composite oxides that can reduce the initial resistance of lithium ion secondary batteries.

FIG. 1 is a cross-sectional view showing a schematic configuration of a lithium ion secondary battery using a fired product of particles according to one embodiment of the present invention. FIG. 2 is a schematic exploded view showing the configuration of a wound electrode body of the lithium ion secondary battery of FIG. 1 . 1 is a backscattered electron image of particles of Comparative Example 2. 1 is a backscattered electron image of the particles of Example 1.

Below, the embodiments of the present invention will be described with reference to the drawings. Note that matters not mentioned in this specification but necessary for implementing the present invention can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and the technical common sense in the relevant field. In addition, in the following drawings, components and parts that perform the same function are explained by using the same reference numerals. Also, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships.

In this specification, the term "secondary battery" refers to an electricity storage device that can be repeatedly charged and discharged, and includes so-called storage batteries and electricity storage elements such as electric double-layer capacitors. In addition, in this specification, the term "lithium ion secondary battery" refers to a secondary battery that uses lithium ions as a charge carrier and realizes charging and discharging by the transfer of charge associated with lithium ions between the positive and negative electrodes.

The particles according to this embodiment are particles that are converted into a lithium transition metal composite oxide by sintering. Each particle contains lithium and a transition metal element. The average particle diameter of the particles is 2.0 μm or more. When a backscattered electron image of the particles is obtained using a scanning electron microscope, the maximum diameter of the domain that is recognized as the brightest area within the particle is less than 1 μm.

The particles according to this embodiment can be used as a positive electrode active material of a lithium ion secondary battery by being converted into a lithium transition metal composite oxide by calcination. Therefore, the particles according to this embodiment can also be said to be lithium-containing precursor particles of the positive electrode active material of a lithium ion secondary battery. The particles according to this embodiment are preferably particles that are converted into a lithium transition metal composite oxide with a layered rock salt crystal structure by calcination.

The particles according to this embodiment are typically particles of a compound that is converted to a lithium transition metal composite oxide when heat-treated for at least one hour at a temperature between 600°C and 1000°C. Examples of such compounds include hydroxides, carbonates, sulfates, nitrates, and oxalates. The particles according to this embodiment are typically hydroxides or carbonates.

The particles according to this embodiment contain Li and a transition metal element within each particle. Therefore, the particles according to this embodiment are different from a mixture of particles containing Li and particles containing a transition metal element. The inclusion of Li and a transition metal element within a single particle can be confirmed by a known method (e.g., Auger electron spectroscopy (AES) analysis using a scanning electron microscope equipped with an Auger electron spectrometer).

The transition metal element contained in the particles is not particularly limited, and examples thereof include Zr, Mo, Co, Fe, Ni, Mn, Cu, Cr, V, Nb, Pt, Pd, Ru, Rh, Au, Ag, Ti, Nb, and W. From the viewpoint of the various characteristics of a lithium ion secondary battery using the fired product of the particles according to this embodiment, the transition metal contained in the particles preferably contains at least one transition metal element selected from the group consisting of Ni, Co, and Mn, and more preferably contains at least Ni, Co, and Mn.

The particles according to this embodiment typically contain oxygen, and may further contain carbon or hydrogen. As described above, the particles according to this embodiment are typically hydroxides or carbonates, and when the particles according to this embodiment are hydroxides, they further contain oxygen and hydrogen. When the particles according to this embodiment are carbonates, they further contain oxygen and carbon.

The particles according to this embodiment may further contain metal elements other than transition metals such as Na, Mg, Ca, Al, Zn, and Sn, metalloid elements such as B, Si, and P, and nonmetallic elements such as S, F, Cl, Br, and I.

Since the initial resistance of a lithium ion secondary battery using the fired product of the particles according to this embodiment is particularly small, the atomic ratio of lithium to the transition metal element in the particles according to this embodiment is preferably 0.75 or more and 1.25 or less, more preferably 0.80 or more and 1.20 or less, and even more preferably 1.05 or more and 1.10 or less.

The types of elements contained in the particles according to this embodiment can be confirmed by known methods (e.g., inductively coupled plasma (ICP) optical emission spectroscopy, etc.). The atomic ratio of lithium to transition metal elements can also be confirmed by known methods (e.g., inductively coupled plasma (ICP) optical emission spectroscopy, etc.).

The particles according to this embodiment are typically composed of secondary particles formed by agglomeration of primary particles, but may be composed of primary particles. The average particle size of the particles according to this embodiment is 2.0 μm or more, and preferably 3.0 μm or more. The upper limit of the average particle size of the particles according to this embodiment is not particularly limited, but is, for example, 25.0 μm or less, and preferably 20.0 μm or less. The average particle size of the particles can be obtained by measuring the particle size distribution of the particles on a volume basis by a laser diffraction scattering method, and determining the particle size at which the cumulative frequency in the particle size distribution is 50% by volume (i.e., the median size: D50).

When a backscattered electron image of the particle according to this embodiment is obtained using a scanning electron microscope (SEM), the maximum diameter of the domain that is recognized as the brightest area (typically a white area) within the particle is less than 1 μm.

In the case of backscattered electron images taken with an SEM, the emission of backscattered electrons from a sample irradiated with an electron beam depends on the average atomic number of the materials that make up the sample, with the amount of electrons emitted increasing as the atomic number increases. Therefore, in a backscattered electron image, areas that contain a large number of elements with small atomic numbers appear dark, and areas that contain a large number of elements with large atomic numbers appear bright.

When the distribution of Li (in other words, the dispersion state) is non-uniform within a particle containing Li and a transition metal element, a region where the transition metal element is concentrated is generated, and in the backscattered electron image, this region forms the brightest region within the particle. In this specification, this region is referred to as a "domain." Therefore, the size of this domain is closely related to the dispersion state of Li within the particle.

In this embodiment, the maximum diameter of the domain being less than 1 μm means that Li is highly dispersed within the particle. When such particles are sintered to obtain a lithium transition metal composite oxide and used as a positive electrode active material for a lithium ion secondary battery, the initial resistance of the lithium ion secondary battery can be reduced. The reason for this is believed to be as follows. When particles with a maximum diameter of this domain being less than 1 μm, i.e., particles in which Li is highly dispersed within the particle, are sintered, the reaction between the Li-containing domain and the transition metal-containing domain is promoted, and a lithium transition metal composite oxide in which the arrangement of Li and the transition metal element is improved (specifically, the Li site occupancy rate is high) can be obtained. This improved arrangement of Li and the transition metal element contributes to lowering the resistance of the lithium ion secondary battery.

In addition, the particles according to this embodiment contain Li and a transition metal element. Therefore, in the conventional method for producing a positive electrode active material, a hydroxide or carbonate of a positive electrode active material precursor is mixed with a Li salt that has been pretreated by pulverization, drying, or the like, and then fired. However, when the particles according to this embodiment are used, there is no need to mix in a Li salt, and the positive electrode active material can be produced easily. In other words, the particles according to this embodiment have excellent productivity for the positive electrode active material.

The particles according to this embodiment may be crystalline or amorphous, and are preferably amorphous. When the particles according to this embodiment are amorphous, when the fired product of the particles is used in a lithium ion secondary battery, the resistance increase during repeated charging and discharging of the lithium ion secondary battery can be suppressed. Therefore, the durability of the lithium ion secondary battery is increased. This is thought to be because the particles are amorphous, which facilitates rearrangement of the transition metal elements during firing, resulting in a lithium transition metal composite oxide with less compositional unevenness. When the particles according to this embodiment are carbonates, the particles can be amorphous. On the other hand, when the particles according to this embodiment are hydroxides, the particles can be crystalline.

The crystallinity of the particles can be confirmed by measuring the particles using X-ray diffraction (XRD) and observing the peak of the (003) plane of the R3m phase derived from the metal hydroxide in the X-ray diffraction spectrum. If the particles are crystalline, a sharp peak is observed in the (003) plane of the R3m phase. If the particles are amorphous, the peaks form a broad halo pattern in the X-ray diffraction spectrum.

Next, the method for producing particles according to this embodiment will be described. The method for producing particles according to this embodiment uses a coprecipitation method, and can be divided into the following three methods (a) to (c).

That is, the manufacturing method includes a step of preparing a mixed solution in which a water-soluble salt of lithium, a water-soluble salt of a transition metal, an ammonium ion donor, and a basic compound are dissolved (hereinafter also referred to as a "mixed solution preparation step S1"), and a step of precipitating particles containing lithium and a transition metal from the mixed solution (hereinafter also referred to as a "precipitation step S2"). Here, in method (a), the basic compound is an alkali metal hydroxide or an alkaline earth metal hydroxide, and the molar ratio of lithium to the transition metal in the mixed solution is 65 to 105. In method (b), the basic compound is an alkali metal carbonate or an alkaline earth metal carbonate, and the molar ratio of lithium to the transition metal in the mixed solution is 5.0 to 8.5. In method (c), the basic compound is an alkali metal carbonate or an alkaline earth metal carbonate, and in the precipitation step A2, a water-soluble organic solvent having a solubility parameter δ of 13.0 or less and P' of 3.5 or more is added to the mixed solution. First, the mixed solution preparation step S1 will be described.

As the Li source, a water-soluble salt of lithium is used. Examples of water-soluble salts of lithium include lithium sulfate, lithium nitrate, and lithium halides (e.g., lithium chloride). The water-soluble salt of lithium may be a hydrate. Among these, lithium sulfate and lithium chloride are preferred, since they can be derived from resources, and lithium sulfate is more preferred.

As the transition metal source, a water-soluble salt of a transition metal is used. Examples of the water-soluble salt of a transition metal include sulfates, nitrates, and oxalates of the transition metal, and among these, sulfates of the transition metal are preferred. The water-soluble salt of a transition metal may be a hydrate.

Examples of the ammonium ion donor include ammonium hydroxide (NH ₄ OH), ammonium sulfate, ammonium nitrate, ammonium halides, and aqueous ammonia. Among these, ammonium hydroxide is preferred.

In method (a), a hydroxide of an alkali metal or an alkaline earth metal is used as the basic compound. This results in crystalline hydroxide particles. Examples of hydroxides of alkali metals or alkaline earth metals include sodium hydroxide, potassium hydroxide, calcium hydroxide, etc., and among these, sodium hydroxide is preferred.

In methods (b) and (c), an alkali metal or alkaline earth metal carbonate is used as the basic compound. This results in amorphous carbonate particles. Examples of alkali metal or alkaline earth metal carbonate include sodium carbonate, potassium carbonate, calcium carbonate, etc., and among these, sodium carbonate is preferred.

As water, ion-exchanged water, distilled water, ultrafiltered water, reverse osmosis water, etc. can be preferably used to prevent the inclusion of impurities.

In the mixed solution preparation step S1, for example, these raw materials are mixed in a container. The container may be a container, a reactor, or the like used in a known coprecipitation method. There is no particular restriction on the order in which the water-soluble salt of lithium, the water-soluble salt of a transition metal, ammonium hydroxide, the hydroxide of an alkali metal or an alkaline earth metal, and water are added. The water-soluble salt of lithium, the water-soluble salt of a transition metal, ammonium hydroxide, and the hydroxide of an alkali metal or an alkaline earth metal may each be added to the container in the form of a solid or in the form of an aqueous solution.

If the molar ratio of lithium to transition metal (Li/transition metal) in the mixed solution is too small, the amount of Li contained in the resulting particles will be too small, and the resulting particles will not have a Li/transition metal molar ratio suitable for a positive electrode active material. On the other hand, if the molar ratio of lithium to transition metal (Li/transition metal) in the mixed solution is too high, the precipitated particles will be contaminated with precipitated lithium salt particles. The molar ratio of lithium to transition metal is equal to the molar concentration ratio of lithium to the molar concentration of the transition metal.

In method (a), Li is used in large excess. Therefore, the molar ratio (Li/transition metal) is 65 or more and 105 or less. From the viewpoint of further reducing the resistance of a lithium ion secondary battery using the fired product of the particles according to this embodiment, the molar ratio (Li/transition metal) is preferably 80 or more and 100 or less, and more preferably 85 or more and 95 or less.

In method (b), Li is used in excess. The molar ratio (Li/transition metal) is 5.0 or more and 8.5 or less. From the viewpoint of further reducing the resistance of a lithium ion secondary battery using the fired product of the particles according to this embodiment, the molar ratio (Li/transition metal) is preferably 6.0 or more and 8.0 or less, and more preferably 6.5 or more and 7.5 or less.

In method (c), a water-soluble organic solvent is used in the precipitation step S2. This allows the amount of water-soluble lithium salt used to be reduced compared to method (b). Therefore, in method (c), the molar ratio (Li/transition metal) may be smaller than that in method (b) and is set appropriately depending on the type and amount of the water-soluble organic solvent. The molar ratio (Li/transition metal) is, for example, 2.0 or more and 7.5 or less, and preferably 3.0 or more and 6.0 or less.

The concentration of the transition metal in the mixed solution is not particularly limited. In method (a), the concentration is, for example, 0.070 mol/L or more and 0.12 mol/L or less. In methods (b) and (c), the concentration is, for example, 0.70 mol/L or more and 1.2 mol/L or less. The concentration of the water-soluble salt of lithium in the mixed solution may be determined from the concentration of the transition metal in the mixed solution and the above molar ratio (Li/transition metal).

The ammonium ion donated from the ammonium ion donor forms a complex with the transition metal. The concentration of the ammonium ion donor is preferably such that the molar ratio of ammonium ion to the transition metal (NH ₄ ⁺ /transition metal) is about 0.5 (e.g., 0.1 to 1).

The concentration of the basic compound is preferably 1.0 mol/L or more and 10.0 mol/L or less, and more preferably 3.0 mol/L or more and 7.0 mol/L or less.

Next, the precipitation step S2 will be described. The precipitation step S2 can be carried out by subjecting the mixed liquid to a crystallization reaction. For example, the precipitation step S2 can be carried out by stirring the mixed liquid for a predetermined period of time. Stirring can promote the crystallization reaction.

In method (c), a water-soluble organic solvent having a solubility parameter δ of 13.0 or less and P' of 3.5 or more is added to the mixed solution in the precipitation step S2. On the other hand, in methods (a) and (b), this water-soluble organic solvent is not basically added. In methods (a) and (b), the mixed solution prepared in step S1 is usually directly subjected to the precipitation step S2.

In method (c), by adding a water-soluble organic solvent with a solubility parameter δ of 13.0 or less and P' of 3.5 or more, when the calcined product of the obtained particles (i.e., lithium transition metal composite oxide) is used in a lithium ion secondary battery, the initial resistance of the lithium ion secondary battery can be significantly reduced. This is thought to be because the water-soluble organic solvent promotes the nucleation of Li, making it possible to reduce the size of the domains containing Li.

Examples of the water-soluble organic solvent include methanol, ethanol, isopropyl alcohol, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, and dioxane. Among these, methanol, ethanol, and tetrahydrofuran are preferred, and ethanol is more preferred. The values of the solubility parameters δ and P' can be found in handbooks and the like.

The amount of the water-soluble organic solvent added is not particularly limited. The water-soluble organic solvent is added so that the concentration in the mixed solution is preferably 1 mol/L or more and 50 mol/L or less, more preferably 5 mol/L or more and 40 mol/L or less, and even more preferably 10 mol/L or more and 30 mol/L or less.

The timing of adding the water-soluble organic solvent to the mixture is not particularly limited, and may be before, during, or after the crystallization reaction. The water-soluble organic solvent also promotes nucleation of the transition metal component. Therefore, from the viewpoint of promoting nucleation of only the Li component, the water-soluble organic solvent is preferably added after the crystallization reaction. After the addition of the water-soluble organic solvent mixture, it is preferable to precipitate the Li component for preferably 10 minutes or more and 5 hours or less (particularly 30 minutes or more and 3 hours or less).

The reaction temperature is not particularly limited. From the viewpoint of the solubility of the raw materials used, it is preferably 10°C or higher, and more preferably 25°C or higher. On the other hand, from the viewpoint of suppressing the volatilization of _NH3 , it is preferably 50°C or lower, and more preferably 35°C or lower. The reaction time may be appropriately determined depending on the concentration of the transition metal in the mixed liquid, the reaction temperature, and the like. The reaction time is preferably 0.5 hours or more and 72 hours or less, and more preferably 1 hour or more and 24 hours or less.

The deposition process S2 may be carried out in air or in an inert gas atmosphere such as nitrogen gas or argon gas.

By carrying out the precipitation step S2, the particles according to this embodiment can be obtained as a precipitate. The precipitate can be collected according to a known method. For example, the precipitate can be collected by a solid-liquid separation method such as filtration, washed with water, etc., and then dried to obtain the particles according to this embodiment.

The particles according to this embodiment can be converted into a lithium transition metal composite oxide by firing. This lithium transition metal composite oxide can be suitably used as a positive electrode active material for lithium ion secondary batteries.

The firing can be carried out in a known firing furnace, for example, by heat treatment at a temperature of 600°C or higher and 1000°C or lower for 1 hour or longer (e.g., 1 hour or higher and 48 hours or lower, preferably 5 hours or higher and 24 hours or lower).

From the viewpoint of increasing the crystallinity of the resulting oxide, it is preferable to perform the calcination in an oxygen-containing atmosphere. The oxygen-containing atmosphere is, for example, an air atmosphere or a gas atmosphere containing oxygen. The oxygen concentration of the atmosphere is preferably 10% by volume or more and 100% by volume or less, and more preferably 18% by volume or more and 100% by volume or less.

When a lithium ion secondary battery is constructed using the lithium transition metal composite oxide obtained by firing, the lithium ion secondary battery has a small initial resistance and therefore has excellent output characteristics. Below, a specific configuration example of a lithium ion secondary battery will be described with reference to the drawings.

The lithium-ion secondary battery 100 shown in FIG. 1 is a sealed battery constructed by housing a flat wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat rectangular battery case (i.e., an outer container) 30. The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin-walled safety valve 36 that is set to release the internal pressure when the internal pressure of the battery case 30 rises to a predetermined level or higher. The positive and negative terminals 42, 44 are electrically connected to the positive and negative current collector plates 42a, 44a, respectively. The material of the battery case 30 is, for example, a lightweight metal material with good thermal conductivity, such as aluminum.

As shown in Figures 1 and 2, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are stacked with two long separator sheets 70 interposed therebetween and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (both sides here) of a long positive electrode collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (both sides here) of a long negative electrode collector 62. The positive electrode active material layer non-forming portion 52a (i.e., the portion where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed) and the negative electrode active material layer non-forming portion 62a (i.e., the portion where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed) are formed so as to protrude outward from both ends of the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction) of the wound electrode body 20. The positive electrode active material layer non-forming portion 52a and the negative electrode active material layer non-forming portion 62a are respectively joined to the positive electrode current collector 42a and the negative electrode current collector 44a.

The positive electrode current collector 52 constituting the positive electrode sheet 50 may be, for example, aluminum foil. The positive electrode active material layer 54 contains at least a lithium transition metal composite oxide, which is a sintered product of the particles according to the present embodiment, as a positive electrode active material. The positive electrode active material layer 54 may further contain a conductive material, a binder, and the like. As the conductive material, for example, carbon black such as acetylene black (AB) or other carbon materials (such as graphite) may be suitably used. As the binder, for example, polyvinylidene fluoride (PVDF) may be used.

The negative electrode current collector 62 constituting the negative electrode sheet 60 may be, for example, copper foil. The negative electrode active material layer 64 includes a negative electrode active material. As the negative electrode active material, for example, a carbon material such as graphite, hard carbon, or soft carbon may be used. The negative electrode active material layer 64 may further include a binder, a thickener, etc. As the binder, for example, styrene butadiene rubber (SBR) may be used. As the thickener, for example, carboxymethyl cellulose (CMC) may be used.

As the separator 70, various porous sheets similar to those conventionally used in lithium ion secondary batteries can be used, and examples thereof include porous resin sheets made of resins such as polyethylene (PE) and polypropylene (PP). Such porous resin sheets may have a single layer structure or a multi-layer structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). The separator 70 may also have a heat-resistant layer (HRL).

The non-aqueous electrolyte can be the same as that used in conventional lithium ion secondary batteries, and typically, a supporting salt can be contained in an organic solvent (nonaqueous solvent). As the non-aqueous solvent, aprotic solvents such as carbonates, esters, and ethers can be used. Among them, carbonates are preferred because they have a particularly high effect of reducing low-temperature resistance due to the positive electrode material. Examples of carbonates include ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). Such non-aqueous solvents can be used alone or in appropriate combination of two or more. As the supporting salt, for example, lithium salts such as LiPF ₆ , LiBF ₄ , lithium bis(fluorosulfonyl)imide (LiFSI) can be suitably used. The concentration of the supporting salt is preferably 0.7 mol/L or more and 1.3 mol/L or less.

The nonaqueous electrolyte may contain various additives such as gas generating agents, film forming agents, dispersants, thickeners, etc., other than the nonaqueous solvent and supporting salt described above, so long as the effects of the present invention are not significantly impaired.

The lithium ion secondary battery 100 has the advantages of low initial resistance and excellent output characteristics. The lithium ion secondary battery 100 can be used for various applications. Suitable applications include a driving power source mounted on vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). The lithium ion secondary battery 100 can also be used as a storage battery for small power storage devices and the like. The lithium ion secondary battery 100 can also be used in the form of a battery pack, typically consisting of multiple batteries connected in series and/or parallel.

Above, a square lithium ion secondary battery having a flat wound electrode body has been described as an example. However, the lithium transition metal composite oxide, which is a sintered product of the particles according to this embodiment, can also be used in other types of lithium ion secondary batteries according to a known method. For example, a lithium ion secondary battery having a stacked electrode body (i.e., an electrode body in which multiple positive electrodes and multiple negative electrodes are alternately stacked) can be constructed using the lithium transition metal composite oxide, which is a sintered product of the particles according to this embodiment. In addition, a cylindrical lithium ion secondary battery, a coin-type lithium ion secondary battery, a laminated lithium ion secondary battery, etc. can also be constructed using the lithium transition metal composite oxide, which is a sintered product of the particles according to this embodiment. Furthermore, an all-solid-state secondary battery using a solid electrolyte can also be constructed.

The following describes examples of the present invention, but is not intended to limit the present invention to those examples.

<Preparation of particles>
Example 1
In a container, _lithium sulfate _{monohydrate (Li2SO4.H2O )} , a hydrate of a transition metal sulfate, sodium hydroxide (NaOH), ammonium hydroxide ( _NH4OH ), and ion-exchanged water were mixed so that the concentration of lithium sulfate was 3.7 mol/L, the concentration of the transition metal sulfate was 0.083 mol/L, the concentration of NaOH was 5.1 mol/L, and the concentration of _NH4OH was 0.042 mol/L. In each of the examples and comparative examples, a mixture containing nickel sulfate hexahydrate, cobalt sulfate heptahydrate, and manganese sulfate pentahydrate in a molar ratio of Ni:Co:Mn=1:1:1 was used as the hydrate of the transition metal sulfate.

The crystallization reaction was carried out for 5 hours, and the precipitate was collected by filtration. The precipitate was washed with ion-exchanged water on the funnel, and then further washed by immersion in water. The precipitate was again collected by filtration, and the precipitate was dried. In this way, the particles of Example 1 were obtained.

Example 2
The particles of Example 2 were obtained in the same manner as in Example 1, except that the above mixing was performed so that the concentration of lithium sulfate was 3.7 mol/L, the concentration of transition metal sulfate was 0.080 mol/L, the concentration of NaOH was 5.1 mol/L, and the concentration of NH ₄ OH was 0.040 mol/L.

Example 3
The particles of Example 3 were obtained in the same manner as in Example 1, except that the above mixing was performed so that the concentration of lithium sulfate was 3.7 mol/L, the concentration of transition metal sulfate was 0.11 mol/L, the concentration of NaOH was 5.1 mol/L, and the concentration of NH ₄ OH was 0.055 mol/L.

Example 4
The particles of Example 4 were obtained in the same manner as in Example 1, except that the above mixing was performed so that the concentration of lithium sulfate was 3.8 mol/L, the concentration of transition metal sulfate was 0.074 mol/L, the concentration of NaOH was 5.1 mol/L, and the concentration of NH ₄ OH was 0.037 mol/L.

Example 5
In a container, _lithium sulfate _{monohydrate (Li2SO4.H2O )} , a hydrate of a transition metal sulfate, sodium carbonate ( _Na2CO3 ), ammonium hydroxide ( _NH4OH ), and ion-exchanged water were mixed so that _the concentration of lithium sulfate was 3.0 mol/L, the concentration of the transition metal sulfate was 0.85 mol/L, the concentration of _Na2CO3 was 5.1 _mol /L, and the concentration of _NH4OH was 0.43 mol/L.

The crystallization reaction was carried out for 5 hours, and the precipitate was collected by filtration. The precipitate was washed with ion-exchanged water on the funnel, and then further washed by immersion in water. The precipitate was again collected by filtration, and the precipitate was dried. This resulted in the particles of Example 5.

Example 6
_Lithium sulfate _{monohydrate (Li2SO4.H2O )} , a hydrate of a transition metal sulfate, sodium carbonate ( _Na2CO3 ), ammonium hydroxide ( _NH4OH ), and ion-exchanged water were mixed in a container so that the concentration of lithium sulfate was 2.4 mol/L, the concentration of transition metal sulfate was 1.4 mol _/ L, the concentration of _Na2CO3 was 5.1 mol/ _L , and the concentration of _NH4OH was 0.7 mol/L.

The crystallization reaction was carried out for 5 hours, and ethanol was added to a concentration of 20 mol/L. The precipitate was then collected by filtration. The precipitate was washed with ion-exchanged water on the funnel, and then further washed by immersion in water. The precipitate was again collected by filtration, and the precipitate was dried. This resulted in the particles of Example 6.

Example 7
_Lithium sulfate _{monohydrate (Li2SO4.H2O )} , a hydrate of a transition metal sulfate _, sodium carbonate ( _Na2CO3 ), ammonium hydroxide ( _NH4OH ), and ion-exchanged water were mixed in a container so that the concentration of lithium sulfate was 2.9 mol/L, the concentration of transition metal sulfate was 0.9 mol/L, the concentration of _Na2CO3 was 5.1 mol/ _L , and the concentration of _NH4OH was 0.45 mol/L.

The crystallization reaction was carried out for 5 hours, and tetrahydrofuran (THF) was added to a concentration of 13 mol/L. The precipitate was then collected by filtration. The precipitate was washed with ion-exchanged water on the funnel, and then further washed by immersion in water. The precipitate was again collected by filtration, and the precipitate was dried. In this way, the particles of Example 7 were obtained.

Example 8
Particles of Example 8 were obtained in the same manner as in Example 7, except that methanol was added at a concentration of 29 mol/L instead of tetrahydrofuran at a concentration of 13 mol/L.

Comparative Example 1
In a container, a transition metal sulfate hydrate, sodium hydroxide (NaOH), ammonium hydroxide (NH ₄ OH), and ion-exchanged water were mixed, and a crystallization reaction was carried out for 5 hours, and the precipitate was collected by filtration. The precipitate was washed with ion-exchanged water on the funnel, and then further washed by immersion in water. The precipitate was collected again by filtration, and the precipitate was dried. Thus, the particles of Comparative Example 1 were obtained.

Comparative Example 2
_Lithium carbonate ( _Li2CO3 ), nickel hydroxide (Ni(OH) ₂ ), cobalt carbonate ( _COCO3 ), and manganese carbonate ( _MnCO3 ) were dispersed in water to obtain a raw material dispersion liquid. This raw material dispersion liquid was spray-dried using a spray dryer to obtain particles.

The average particle size of the particles obtained in each example and comparative example was approximately 5 μm.

<AES-SEM Evaluation>
Using a scanning electron microscope equipped with an Auger electron spectrometer, AES analysis was performed on the particles obtained in each Example and Comparative Example. It was confirmed that Li and transition metal elements (Ni, Co, Mn) coexisted in one particle in the particles of Comparative Example 2 and each Example.

<SEM Evaluation>
A backscattered electron image was obtained using an SEM for the particles obtained in each Example and Comparative Example. The maximum diameter of the domain, which was understood as the brightest area in the particle, was evaluated. The results are shown in Table 1. For reference, the backscattered electron images of Comparative Example 2 and Example 1 are shown in Figures 3 and 4, respectively. The parts indicated by the arrows correspond to the domains. In the backscattered electron image of Comparative Example 2 (Figure 3), it can be seen that white domains with a maximum diameter far exceeding 1 μm have been formed. On the other hand, in the backscattered electron image of Example 1 (Figure 4), it can be seen that many white domains exist in the form of particles, and their size is much smaller than 1 μm.

<ICP analysis>
The particles obtained in each of the Examples and Comparative Examples were subjected to ICP analysis to determine the atomic ratio of Li to the transition metal element. The results are shown in Table 1.

<Crystallinity evaluation by XRD measurement>
XRD measurements were performed on the particles obtained in each Example and Comparative Example to obtain X-ray diffraction spectra. In Comparative Examples 1 and 2 and Examples 1 to 4, a sharp peak was observed on the (003) plane of the R3m phase derived from a metal hydroxide, which was a typical spectrum for crystalline particles. In Examples 5 to 8, the peak was broad across the entire spectrum. This was a typical spectrum for amorphous particles.

<Preparation of Lithium-Ion Secondary Battery for Evaluation>
The particles of Comparative Example 1 were mixed with lithium hydroxide so that the molar ratio of Li/transition metal element was 1.05. This was baked at 850° C. for 10 hours in an oxygen atmosphere to obtain a lithium transition metal composite oxide. The particles of Comparative Example 2 were baked at 900° C. for 10 hours in an oxygen atmosphere to obtain a lithium transition metal composite oxide. The particles of each Example were baked at 850° C. for 10 hours in an oxygen atmosphere to obtain a lithium transition metal composite oxide.

The obtained lithium transition metal composite oxide (positive electrode active material), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in N-methylpyrrolidone (NMP) in a mass ratio of positive electrode active material:AB:PVDF = 85:10:5 to prepare a paste for forming a positive electrode active material layer. This paste was applied to an aluminum foil with a thickness of 15 μm and dried to produce a positive electrode sheet.

A paste for forming the negative electrode active material layer was prepared by mixing natural graphite (C) as the negative electrode active material, styrene butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickener in a mass ratio of C:SBR:CMC = 98:1:1 in ion-exchanged water. This paste was applied to copper foil with a thickness of 10 μm and dried to produce a negative electrode sheet.

In addition, a 20 μm-thick porous polyolefin sheet with a three-layer structure of PP/PE/PP was prepared as a separator sheet.

The positive electrode sheet, the negative electrode sheet, and the separator sheet were stacked, and the electrode terminals were attached and housed in a laminate case. Next, a non-aqueous electrolyte was injected into the laminate case, and the laminate case was sealed airtight. The non-aqueous electrolyte was a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3:4:3, with LiPF ₆ dissolved as a supporting salt at a concentration of 1.0 mol/L. In this manner, lithium ion secondary batteries for evaluation of each of the examples and comparative examples with a capacity of 10 mAh were obtained.

<Initial resistance measurement>
Each evaluation lithium ion secondary battery was adjusted to a SOC (State of charge) of 50% and then placed in an environment of 25° C. Discharge was performed for 10 seconds at a current value of 100 mA, and the voltage value 10 seconds after the start of discharge was measured to calculate the initial battery resistance. Then, the resistance ratio of the other evaluation lithium ion secondary batteries was calculated when the resistance of the evaluation lithium ion secondary battery of Comparative Example 1 was set to 100. The results are shown in Table 1.

<Evaluation of Resistance Increase Rate after Cycles>
Each evaluation lithium ion secondary battery was placed in an environment of 60° C., and 200 cycles of charge and discharge were repeated, with one cycle being a constant current charge at 1C to 4.1 V and a constant current discharge at 1C to 3.0 V. Thereafter, the battery resistance after 200 cycles was determined in the same manner as in the initial resistance measurement. The resistance increase rate (%) was calculated from (battery resistance after 200 charge and discharge cycles/initial resistance)×100. Then, the ratio of the resistance increase rate of the other evaluation lithium ion secondary batteries was calculated when the resistance increase rate of the evaluation lithium ion secondary battery of Comparative Example 1 was set to 100. The results are shown in Table 1. The results are shown in the table.

The results in Table 1 show that the particles disclosed herein can provide particles that can be converted into lithium transition metal composite oxides that can reduce the initial resistance of lithium ion secondary batteries. In addition, when the particles are amorphous, the lithium ion secondary batteries also have excellent durability.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples given above.

That is, the particles and the production method thereof disclosed herein are the following items [1] to [6].
[1] Particles that are converted into a lithium transition metal composite oxide by firing,
The particles contain lithium and a transition metal element within each particle,
The average particle size of the particles is 2.0 μm or more,
When a backscattered electron image of the particle is obtained by a scanning electron microscope, the maximum diameter of a domain recognized as the brightest region in the particle is less than 1 μm.
particle.
[2] The particle according to item [1], wherein the atomic ratio of lithium to the transition metal element is 0.75 to 1.25.
[3] The particle according to item [1] or [2], wherein the particle is a hydroxide or a carbonate.
[4] The particle according to any one of items [1] to [3], wherein the particle is amorphous.
[5] The particle according to any one of items [1] to [4], wherein the transition metal element includes at least Ni, Co and Mn.
[6] preparing a mixed solution in which a water-soluble salt of lithium, a water-soluble salt of a transition metal, an ammonium ion donor, and a basic compound are dissolved;
Precipitating particles containing lithium and a transition metal from the mixed solution;
Equipped with
(a) the basic compound is a hydroxide of an alkali metal or an alkaline earth metal, and the molar ratio of lithium to the transition metal in the mixed solution is 65 or more and 110 or less;
(b) the basic compound is a carbonate of an alkali metal or an alkaline earth metal, and the molar ratio of lithium to the transition metal in the mixed solution is 5.0 or more and 8.5 or less, or (c) the basic compound is a carbonate of an alkali metal or an alkaline earth metal, and in the step of precipitating the particles, a water-soluble organic solvent having a solubility parameter δ of 13.0 or less and P' of 3.5 or more is added to the mixed solution.
A method for producing particles.

20 Wound electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector 44 Negative electrode terminal 44a Negative electrode current collector 50 Positive electrode sheet (positive electrode)
52 Positive electrode current collector 52a Positive electrode active material layer non-forming portion 54 Positive electrode active material layer 60 Negative electrode sheet (negative electrode)
62 Negative electrode current collector 62a Negative electrode active material layer non-forming portion 64 Negative electrode active material layer 70 Separator sheet (separator)
100 Lithium-ion secondary battery

Claims (5)

Particles of a lithium transition metal composite hydroxide or a lithium transition metal composite carbonate, the particles being lithium-containing precursor particles of a positive electrode active material ;
The particles contain lithium and a transition metal element within each particle,
The average particle size of the particles is 2.0 μm or more,
When a backscattered electron image of the particle is obtained by a scanning electron microscope, the maximum diameter of a domain recognized as the brightest region in the particle is less than 1 μm.
particle. The particle according to claim 1, wherein the atomic ratio of lithium to the transition metal element is 0.75 or more and 1.25 or less. The particle of claim 1, wherein the particle is amorphous. The particle according to claim 1, wherein the transition metal elements include at least Ni, Co and Mn. A step of preparing a mixed solution in which a water-soluble salt of lithium, a water-soluble salt of a transition metal, an ammonium ion donor, and a basic compound are dissolved;
Precipitating particles containing lithium and a transition metal from the mixed solution;
Equipped with
(a) the basic compound is a hydroxide of an alkali metal or an alkaline earth metal, and the molar ratio of lithium to the transition metal in the mixed solution is 65 or more and 110 or less;
(b) the basic compound is a carbonate of an alkali metal or an alkaline earth metal, and the molar ratio of lithium to the transition metal in the mixed solution is 5.0 or more and 8.5 or less, or (c) the basic compound is a carbonate of an alkali metal or an alkaline earth metal, and in the step of precipitating the particles, a water-soluble organic solvent having a solubility parameter δ of 13.0 or less and P' of 3.5 or more is added to the mixed solution.
A method for producing particles.

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